Best Practice: Thermal Management of Racks

By Peter Aylett, Archimedia Middle East.
In our new Dubai experience centre, the rack building and engineering space is the first room...

By Peter Aylett, Archimedia Middle East.
In our new Dubai experience centre, the rack building and engineering space is the first room that people see when they walk through the front door. It has a whiff of Swiss watchmaking factory about it, and has been positioned so that the first thing we discuss with potential clients is that we are engineers, and not simply retailers of electronic toys.
Although building and wiring a rack is as much art as it is science, and demands a considerable level of experience, thought and patience from the technician, one element of pure engineering is correct thermal management. Once digital equipment reaches around 30ºC, every 5ºC rise reduces its lifespan by around 40%. This specially true of anything that contains a hard disk drive.
According to the CEA/CEDIA-CEB24 Home Theater HVAC Bulletin http://cedia.net/ProductCatalog/ProductCategory.aspx?ID=31, when we consider that around 99.6% of the power that goes into a rack gets converted into heat, managing this heat is an essential element in ensuring that equipment is reliable and long-lived.
The following are five tips to help you do this.
Know your BTUs
A British Thermal Unit (BTU) is the amount of heat required to raise the temperature of 1 pound (0.454 kg) of liquid water by 1°F (0.56°C). BTUs are the measurement used within the air-conditioning industry to describe heat energy. When a rack is in an enclosed space, heat energy generated by equipment must be removed from the space in order to stop it from heating up beyond an acceptable level.
All equipment in a rack will generate heat to a greater (class A amplifiers) and lesser (class D amplifiers) degree. When heat generated is less than or equal to 1700 BTU/H, passive cooling (or natural convection) can be used. According to the CEA/CEDIA-CEB24 Home Theater HVAC Bulletin http://cedia.net/ProductCatalog/ProductCategory.aspx?ID=31, when heat generated is greater than 1700BTU/H, active cooling using fans, or air conditioning, should be used.
Some equipment manufacturers specify the BTU output of their equipment in the specifications. For those that do not, there is an excellent free white paper published by Middle Atlantic Products http://www.middleatlantic.com/resources/white-papers.aspx that explains how to calculate it.
Use Thermostatically-controlled Fans
Fans are noisy and draw dust and dirt into equipment spaces. For these reasons, always install thermostat control. An excellent way of measuring the effectiveness of rack cooling is to purchase some minimum/maximum digital thermometers designed to be used in children's nurseries. These can be left in the rack (near the top) for a few days and will tell you the temperature extremes that the inside of the rack got to. If the temperature is still too high, try turning down the temperature on the thermostat at which the fan comes on at.
[caption id="attachment_6845" align="aligncenter" width="493"] Thermostatically-controlled fans reduce power consumption, noise and dust.[/caption]
Use the Correct Fan
The CEA/CEDIA-CEB24 Home Theater HVAC Bulletin http://cedia.net/ProductCatalog/ProductCategory.aspx?ID=31 discusses how to calculate fan CFM (Cubic Feet per Minute) based upon the BTU value of your equipment. Fans can be noisy, so try to use a large, slow fan rather than a small, highly-revving one. Once you have done the calculations, do not over-specify fan CFM, as this will draw more dust and dirt into the equipment space. Consider using filters on the air intakes, but ensure that these are checked and cleaned regularly to maintain airflow. This is a great opportunity to discuss a maintenance contract with the customer!
Manage Airflow
Fans installed at the top of racks or cabinets that have open sides will simply churn the air around near the fan and will not draw in cool air. The rule is to place fans at the top and to draw in cool air at the bottom which then gets sucked out by the fan at the top aided by the natural convection of warm air rising. Air will take the path of least resistance so when using fans it is important to manage airflow.
[caption id="attachment_6847" align="aligncenter" width="600"] Poor rack design can lead to fans being almost useless.[/caption]
Beware of Wrongly-specified Air Conditioning
If you have the luxury of having your equipment space air-conditioned, here are some tips -
1) Ensure that you fit a humidistat as well as a thermostat. Humidity is as much of an equipment killer as temperature - if you cool a humid space down quickly you will get condensation forming on equipment which will quickly damage it. Similarly, low humidity can lead to static build up as this charge is not dissipated due to lack of moist air. Relative humidity of the space at around 25ºC should be kept between 30-60%.
[caption id="attachment_6846" align="aligncenter" width="600"] Most air-conditioning systems can be controlled using a humidistat that senses relative humidity in addition to temperature.[/caption]
2) Install a temperature monitoring system that will automatically alert you if the temperature gets above 30ºC. Equipment rooms are rarely visited by homeowners, so it is possible that the air-conditioning unit could fail and no one would find out. Ensure that the customer has a maintenance contract with the air-conditioning company to ensure that units are checked and filters are changed regularly.
3) In an air-conditioned space, you will not require panels on the sides or back of the rack, or fans at the top.
Conclusion
Many companies put a huge amount of effort into making racks look beautiful. The CEDIA Awards 'Best Dressed Rack' category is testament to this. Please ensure, however, that thermal management is considered as just as important, so that you ensure that the installed equipment is as reliable and long-lasting as possible.
Peter Aylett is a world-renowned speaker and lecturer in residential technology, and the Technical Director at Archimedia, a multinational high-end residential integrator in The Middle East. He is also currently Chair of CEDIA’s International Technology Council Applied Content Action Team, and a regular contributor to HiddenWires.www.hometechassoc.com
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